Polish were crossed with Silkies awhile back to get their crests larger and because Polish have vaulted skulls that is likely where they came from. It probably was originally a mutation and it absolutely serves the chicken no purpose. It seems the genetics of it vary with the line. With my line only the hens had vaulted skulls but now they are showing up on the roosters to a lesser degree.

Yes I think it makes them more prone to hurting their heads. There is a big hole up there where there is normally a skull cap.
But of course you should always be careful of a chick/chickens head.
Some people prize this trait, seems to make for a larger crest.
Makes no difference if you breed vaulted, to a non-vaulted Silkie.
Hope this helps,
Cheryl

yes, some people think that it makes for a larger crest, but I have had non-vaulted chicks get equally as large of a crest. It will make them more prone to injuries as there is no skull on top and its just their little brain underneathe the skin. They also get extremely prone to mites on top of the head and I have never give it much thought till right now but I have some silkies that get mites there a lot and others never get any...wonder if mites feed from the head on vaulted more than non? Hmmmm........

Yes I agree, some people to seem to think that vaulted heads make larger crests, but there are non-vaulted skulls that are fantastic crested birds.[ I know I have some, just like you apparently do]]
The mite situation I don't know about, I am blessed to be mite free for some reason! Maybe the 7 dust???

I found this on Photobucket. It must be a Silkie skull since it is black. The brains are exposed and sometimes bulge from the openings. There are varying degrees of vaulting, to a huge vault to almost non-existent, to everything in between. I can sex my Silkies from the time they hatch since the pullets will have the largest vaults.

This is what happens when people purposely breed freaks of nature.
I have a lovely little Polish chick with a HUGE vaulted skull right now; I love the little bugger to death, but I would NEVER breed more like her.

Are you serious? Pullets have larger vaulted skulls? I have a total of three thats hatched. Two of them have HUGE vaulted skull!!!! I mean really big! I have another without it and it only has 4 toes What should I do with this little chick? Im not gonna bring that bird into my breeding flock!